Understanding HRV
The field guide.
Short, accurate, opinionated answers to the questions people actually ask about heart rate variability.
The pillars
What is HRV, really?
The beat-to-beat variation in your heartbeat, explained without the wearable-marketing fluff.
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What counts as a 'good' HRV?
Published norms by age, and why no single number counts as good.
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How to actually improve your HRV
Five interventions and the evidence behind each. The order is our editorial judgment.
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HRV by age
RMSSD and SDNN by sex and decade from a published dataset, and what those norms can't tell you.
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What moves HRV
How alcohol affects HRV
What the dose-response studies show: more alcohol, lower HRV, and what they didn't measure.
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How sleep affects HRV
How HRV moves with sleep stage, sleep loss and timing, where the evidence is mixed, and where breathing fits.
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How stress shows up in HRV
Acute vs chronic stress produce different HRV signatures. Confusing them leads to wrong interventions.
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HRV and training
Readiness, recovery, and what the HRV-guided training studies found.
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Low HRV: causes ranked
The common reasons HRV drops, in our editorial order by how quickly each can plausibly be reversed.
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RMSSD vs SDNN
Why your Apple Watch HRV doesn't match your Oura HRV. The two metrics, plainly.
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The vagus nerve and HRV
What the vagus nerve does at the heart, what HRV can and can't tell you about it, and the polyvagal debate.
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How to monitor HRV
Two different questions — state-now vs trend — and which device answers each. Hardware spectrum, practical setup, when to stop monitoring daily.
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Find your resonance frequency
Six breaths per minute is a population average, not your number. The clinical assessment protocol, an equipment-free approximation, and a wrist-based version.
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The BOLT score
A breath-hold measured to the first urge, not the limit. It needs no wearable, and it is not a validated test. What the trials do and don't show.
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